In pursuance of the problem of double-length mitochondrial DNA molecules formed in patients with myelogenous leukemia, we plan to examine bone marrow aspirates in order to determine whether those molecules are present in bone marrow as well as in the peripheral leucocytes. This study will be carried out in collaboration with investigators at the City of Hope, Duarte, California. We also plan to examine specific endonuclease fragments obtained from mitochondrial DNA catenanes found in cells in culture in order to determine whether any single region of the DNA in the submolecule is involved in a recombination process in which label is incorporated via repair synthesis. We plan to continue studies of the mechanism of action of superhelix relaxing protein in the nuclei and the mitochondria of human and mouse cells in culture. This enzyme is postulated to play a significant role in the replication of DNA in the nucleus and in mitochondria. We plan to compare the mitochondrial DNA obtained from in vitro constructed fragmets grown in E. coli with corresponding DNA from animal cells in culture. The first problems to be dealt with are the structure of the recombinant, the fidelity of replication, and the possible presence of ribonucleotides and alkali-sensitive sites in the mitochondrial portion of the recombinant genome.